What Types of Rogue Planet Are There?
Contents:
Introduction: 5
How to use this booklet: 6
What we (probably) won’t know about rogue planets for decades or centuries: 7
Locations in interstellar space: 8 Section contents: 9 Section introduction: 10 A note on galactic directions: 10 Galactic nucleus (central bulge and surrounding region): 11 Spiral arms/Galactic disk: 12 Local intergalactic space: 13 Globular clusters: 14 Open clusters: 15 Nebula: 16
Basic rogue planet tropes: 17 Section contents: 18 Section introduction: 19 How to use this section: 19 A note on internal heat, and why rogue planets don’t always freeze: 20 Zombie Rogues (ZR): Worlds that have been dead for billions of years but are still wandering. 21 Dying Rogues (DR): Worlds that are mostly dead - but where a few habitable niches may survive. 23 Cryptic Rogues (CR): Worlds with habitable subsurface oceans under inhospitable surfaces. 25 Habitable Rogues (HR): Worlds with habitable surfaces under thick, insulating, atmospheres 27 Modifiers: 29 Hypervelocity rogues: 29 Double rogue planets: 29 Carbon world: 29
Rogue gas giant planets - and their moon systems: 31 Section contents: 32 Section introduction: 33 How to use this section: 33 Layout of a gas giant’s moon system: 34 Core system: 34 Main system: 35 Outer system: 36 Water moon tropes: 37 Ice moon tropes: 39 Volcanic moon tropes: 41 SampleHabitable surfaced (gas dwarf) moon tropes: file43 Asteroid or comet-like, non spherical, moon tropes 45
2 Gas giant tropes and their moon families: 47 A bit of explanation of radiation belts, and how they relate to rogue gas giant planets: 49
General notes: 50 An average human is… 50 Mass vs gravity for solid worlds: 50 Ring systems, antimatter mines and collection methods: 51 Size vs mass vs gravity for gas giant planets: 53 Planets can heat their moons with just their gravity… 53 Gas giants vs brown dwarf 'sub-stars': 53 Gas giants vs ice giants: 54
Glossary of terms: 55 Radiation countermeasures for space vehicles passing through radiation belts: 55 Deflectors: 55 Barriers: 55 Other kinds of radiation hardening: 56 Impact countermeasures for spacecraft: 56 General terms: 57
Equations used: 61 Introduction: 61
Further reading: 64
Sample file
3 Sample file
4 Introduction: Space, like The Hitchhiker’s Guide to the Galaxy says, is big. Really big. In fact, compared to the vast, cold, wastes between the stars, the oases of light and warmth stars make for planets like ours to orbit in are the tiniest fraction of the universe - like floating candles, separated by vast stretches of dark Antarctic ocean. When a planet gets thrown out of it’s solar system by something it’s truly lost: No star will appear as more than a point of light, and the odds of it getting recaptured by another solar system at all, never mind on a stable orbit, are minute. Some rogue planets are even natives to the interstellar dark - formed independently, from the same cloud of dust and gas that was birthing stars all around them. Those wanderers have never known a sun’s warmth, and likely never will. Does that mean all lost worlds are dead balls of ice and rock, spinning into a glum eternity? No. Not quite. True: They’re never going to make great vacation spots, other than to some niche crowd looking for two weeks of stygian darkness*. But being dark doesn’t have to make them dull - after all, our own Earth has enough internal heat to have powered volcanic activity, which supports life that's independent of the Sun, for billions of years. Add a really thick, insulating, atmosphere and you might have a world warm enough for life to survive, even as it wandered interstellar space. The space around, and views from, rogue worlds may not be as dull as you might expect either: Rogue planets with moons in their skies, even entire moon systems, are possible - and a world with a really big moon would feel tidal forces (see page 53) powerful enough to heat both their cores, keeping their volcanoes glowing. A gas giant could warm an entire system on close packed moons this way - a miniature solar system evolving in darkness. And the variety of rogue worlds is thought to cover gas giants, ice covered ocean worlds, mere balls of rock and worlds with surfaces that could support life beneath super-thick atmospheres. Both the background starfields and radiation levels vary quite a bit around the galaxy too - from the brilliant, rad spitting, pulsar studded skies of the galactic core to the utter darkness and isolation of intergalactic space. in other words there’s plenty for your characters to explore, puzzle over, be put in mortal danger by, or perhaps even call home...
Sample* Having seen the internet I have no doubt that such a crowd exists however. file
5 How to use this booklet: To keep things consistent we’ve broken creating a rogue world, whether a frozen dwarf planet or a seething lava ball, into three steps: Defining the planet itself, modifying it, and deciding where in the galaxy to place it. While the first two might be the most immediate concern for any given scene or gaming session, the second will determine how the world fits into the wider universe, or campaign. The way we suggest proceeding is: 1. Go to the contents section for the 'Tropes' (p 18), which lists some useful, basic, tropes for a wandering interstellar planet. Each of the tropes are divided into sub tropes, and so is presented as a table followed by notes. Each entry on a table gives a basic description of a sub trope: Surface conditions, visuals, likely experiences, hazards and countermeasures. The notes provide more depth. 2. Following the basic tropes are a series of ‘modifiers (p 29)’, which can be added to any trope to give it an unusual property - for example the ‘carbon world’ modifier makes the planet a super-hard ball of diamond and carbides, with oceans of oils instead of water. 3. Once you have your sub-trope set up, look at the types of interstellar space (p 8) section, which will give a visual description and technical details (E.G radiation levels) for each region of the galaxy. The exceptions to this are moons of rogue gas giants - these have a seperate section starting on page 31, as a world that is a gas giant’s moon can be part of a system of many moons, following the giant through space. For how to generate a moon of a gas giant see page 33.
Rogue planets can plausibly turn up in any part of the galaxy, as they wander for billions of years. That said, there are neighbourhoods that certain tropes tend to be more likely in: All tropes are equally likely to be found in the galactic core (p11), spiral arm (p 12), nebula (p 16), and open clusters (p15). Only gas giant planets are common in globular cluster (p14), where the elements for rocky or icy planets are rare. All tropes are rare - but not impossible - to find in intergalactic space, since everything’s rare out there.
After the tropes and modifiers are some general notes covering things like what hitting a solid object (from tiny dust grains up to boulders) will do at various real-space spacecraft speeds, and how increasing a terrestrial planet’s mass increases its surface gravity. Lastly comes the glossary, titles and links to some of the scientific source materials that make for useful further reading, and any equations that might be of use to the really detail oriented.
For those reading through this booklet in order: The various regions of interstellar space are presented first, to provide context for the rogue planet tropes themselves, then the tables of tropes, then the modifiers - using the document that way offers a chance to take a more completist, top down, approach. Sample file
6 What we (probably) won’t know about rogue planets for decades or centuries: If you had something different in mind, location or trope wise, from what’s in the tables don't worry - this subject has lots of wriggle room as what we know, or can reasonably infer, is so limited. Some of the most significant question marks for rogue planets are:
How many are there in our galaxy? Depending on which model of planet growth you use, and where you put the size limits for ‘planet’, estimates run from 50 billion to 20 thousand trillion. Planned space telescopes may narrow the estimates in the next 20 years, but a truly precise count is a long way off.
Where’s the nearest? There’s a lot of scope for roughly Earth-sized (or smaller) wandering planets to be closer to us than the nearest star and have remained undetected, as their frozen surfaces emit no light and little infra-red.
Are there intergalactic rogues? It's an open question whether rogue planets mostly stay within our galaxy, or stray into intergalactic space: They might get a long way from their usual haunts if they... ● Have a slingshot encounter with a giant black hole or large star, giving it a higher than normal speed. ● Are very ancient and have had more time to wander ● Have been propelled by the blast of a kilonova, supernova, hypernova, gamma ray burster - in which case they’ll be badly damaged.
What fraction of them have moons, or are double planets? Being thrown out of a solar system is traumatic for a planet and its moon(s). Moon systems of ice and gas giants, bound by stronger gravity, would keep together more easily. Around 5% of worlds like Earth might, in principle, hold onto theirs - but all we have are simulations.
Radiation environments: These depend on location in the wider galaxy. A combination of telescope observations and computer models can provide best-guess figures. But only four spacecraft have even reached local interstellar space to take measurements - so those best guesses, which are used in the ‘locations in interstellar space’ section (p 8), are only that.
How long have they wandered? How old a rogue world was when it went rogue, and how long it has been rogue, can heavily impact its current state: A whole planet takes time to cool, so an Earthlike world might remain borderline habitable for weeks, even months, depending on how fast it’s leaving its home system. A world with a thicker, more insulating, atmosphere might keep its heat longer than that - and, if it is already old enough to have developed life, that life might get time to adapt. On the other hand a world that has been wandering for ten billion years might well have cooled all the way to its core, becoming entirely inert.
If you’d like to build something more unique you might find the general notes (p 50), equations used Sample(p61), and further reading (p 64) sections helpful. file
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